1. Field of the Invention
The present invention relates to a measuring apparatus and a measuring method which measure a phase shift amount and a transmittance of a phase shift mask.
2. Description of Related Art
As photomasks designed to handle the miniaturization of LSIs, half-tone type phase shift masks and Levenson type phase shift masks have been put into practical use. The phase shift masks are each provided with a phase shifter that gives a phase difference of π or an odd multiple of π to light passing through adjacent pattern elements of a mask pattern. For example, in the half-tone type phase shift masks, a light shielding pattern formed of molybdenum and silicon films operates as a phase shifter, and a phase difference of π or an odd multiple of π it is introduced between light transmitted through the light shielding pattern and light transmitted through an adjacent opening. In the phase shift masks, the resolution of an exposed pattern decreases along with the deviation of the phase shift amount of a phase shifter from the design value, which leads to deterioration in the quality of a photomask. For this reason, in a photomask production process, the phase shift amount of a phase shifter is one of the important parameters for securing the quality of a phase shifter. The phase shift amount of a phase shifter is measured by using a phase shift amount measuring apparatus.
As a phase shift amount measuring apparatus of a related art, a measuring apparatus that uses two-beam interferometry has been known (for example, see Japanese Unexamined Patent Application Publication No. 2005-83974). In this phase shift amount measuring apparatus of the related art, a monitor pattern of a square phase shifter is formed in a corner portion of a phase shift mask to be inspected, and a coherent light beam emitted from a lighting system is projected toward the monitor pattern. The transmitted beam emitted from the monitor pattern is directed onto a Mach-Zehnder interferometer and is converted into two laterally-shifted beams which are laterally shifted by a predetermined shearing amount. The two laterally-shifted beams form an interference image in which the images of the monitor pattern overlap each other, two interference images in which the image of the monitor pattern and the image of an opening, which are formed on both ends of the interference image, overlap each other, and two interference images in which the images of the opening, which are formed on both ends of the interference images, overlap each other. The interference images formed by the two laterally-shifted beams are picked up by a CCD camera, and the phase shift amount is calculated based on a phase difference between interference lights generated from two interference images in which the light beam transmitted through the monitor pattern and the light beam transmitted through the opening overlap each other.
The phase shift amount and the transmittance of a phase shifter are important physical quantities in the phase shift masks. In other words, the phase shifter functions as a light shielding pattern and operates to specify a resolution and a dose amount in the exposure of light onto a wafer. Accordingly, if the transmittance of the phase shifter is outside of a predetermined range, the resolution and the dose amount in the exposure process fall outside a threshold range, which causes deterioration in the quality of the phase shift mask. For this reason, in the phase shift amount measuring apparatus of the related art, not only the phase shift amount of the phase shifter, but also the transmittance of the light shielding film is measured.
In the phase shift amount measuring apparatus of the related art, the amplitude of the interference fringe that forms the interference image in which the images of the opening overlap each other is measured, and the amplitude of the interference light of the area in which the images of the phase shifter overlap each other is also measured, thereby obtaining the transmittance from the ratio between the two amplitudes (see Japanese Unexamined Patent Application Publication No. 2005-83974).
In the phase shift amount measuring apparatus of the related art, in order to measure the phase shift amount and the transmittance of the phase shifter, it is necessary to form three types of interference images, i.e., an interference image in which the images of the phase shifter overlap each other, two interference images in which the image of the phase shifter and the image of the opening overlap each other, and interference images in which the images of the opening overlap each other. In order to form these three types of interference images, a relatively large monitor pattern is required. This is contrary to the market need for miniaturization of LSIs.
Further, in the phase shift amount measuring apparatus of the related art, the interference images for use in measuring the transmittance of the phase shifter are different from the interference images for use in measuring the phase shift amount of the phase shifter. Therefore, it is necessary to perform the measurement twice while moving a stage. This leads to deterioration in the throughput of the measurement.
An object of the present invention is to realize a phase shift amount measuring apparatus and a phase shift amount measuring method which are capable of measuring a phase shift amount and a transmittance of a phase shifter in one measurement step by using a miniaturized monitor pattern.